On the World Standards website, an explanation on why in some countries they drive on the left. Aside from a variety of historical reasons, it really seems to be a matter of who conquered who: if it was the British, you'd drive on the left, if by the French, on the right.
Despite the developments in the US, some parts of Canada continued to drive on the left until shortly after the Second World War. The territory controlled by the French (from Quebec to Louisiana) drove on the right, but the territory occupied by the English (British Columbia, New Brunswick, Nova Scotia, Prince Edward Island and Newfoundland) kept left. British Columbia and the Atlantic provinces switched to the right in the 1920s in order to conform with the rest of Canada and the USA. Newfoundland drove on the left until 1947, and joined Canada in 1949.
In Europe, the remaining left-driving countries switched one by one to driving on the right. Portugal changed in 1920s. The change took place on the same day in the whole country, including the colonies. Territories, however, which bordered other left-driving countries were exempted. That is why Macau, Goa (now part of India) and Portuguese East Africa kept the old system. East Timor, which borders left-driving Indonesia, did change to the right though, but left-hand traffic was reintroduced by the Indonesians in 1975.
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